1. Field of the Invention
The present invention relates to microwave band-pass filters using microstrip lines and an adjusting method of the filter characteristic, and more particularly to microwave band-pass filters of which miniaturization and improvement of the filter characteristic are possible and a filter characteristic adjusting method thereof.
2. Description of the Background Art
Microwave band-pass filters utilizing the resonance of distributed parameter circuits are frequently used at present in the fields such as the satellite broadcasting, the personal radio. The microwave band-pass filters include two types, the comb line type and the interdigital type.
As shown in FIG. 17, a microwave band-pass filter of comb line type includes a dielectric substrate A, a grounding electrode B formed all over the back surface of the dielectric substrate A, a short-circuit electrode 4 formed on one side in a width direction of the dielectric substrate A, a plurality of resonant lines 11, 12, 13 formed in a length direction of the dielectric substrate A, of which one ends are commonly connected to the short-circuit electrode 4, an input line 2 connected to the resonant line 11 at the first stage among the plural stages of resonant lines, and an output line 3 connected to the resonant line 13 at the last stage among the plural stages of resonant lines. The dielectric substrate A formed of dielectric material having permittivity of about 90, e.g. BaO-Nd.sub.2 O.sub.3 -TiO.sub.2 system material has a width of H. Each resonant line 11, 12, 13 has a length of L and a width of W.
In the above-described structure, the energy of the microwave inputted to the resonant line 11 is imprisoned in the dielectric substrate A to produce a standing wave having 1/4 wave length. Accordingly, when the wave length of the supplied microwave is .lambda..sub.0 and the effective permittivity of dielectric substrate A is .epsilon., the length of a resonant line can be .lambda..sub.0 /4.sqroot..epsilon.. The characteristic impedance Zo of the resonant line is proportional to H/W.
FIG. 18 is a diagram showing a microwave band-pass filter of interdigital type. The microwave band-pass filter includes short-circuit electrodes 41, 42 formed on both sides in a width direction of a dielectric substrate A, resonant lines 11, 13 connected to the short-circuit electrode 41, a resonant line 12 connected to the short-circuit electrode 42, and an input line 2 and an output line 3 connected to the short-circuit electrode 42.
Referring to FIGS. 17 and 18, the comb line type and the interdigital type are different in that one ends of resonant lines of the comb line type are commonly connected to a short-circuit line, but one ends of resonant lines of the interdigital type are alternately connected to short-circuit electrodes 41, 42.
FIG. 19 is a diagram for describing the relationship between a coupling coefficient k.sub.1 between resonant lines of a microwave band-pass filter of comb line type and a coupling coefficient k.sub.2 between resonant lines of a microwave band-pass filter of interdigital type. Here, the coupling coefficient means the strength of inductive coupling between resonant lines. The coupling coefficient k is proportional to an interval d between resonant lines. The coupling coefficient k.sub.1 of a comb line type microwave band-pass filter is larger than the coupling coefficient k.sub.2 of an interdigital type microwave band-pass filter because the directions of electric fields in adjacent intervals between resonant lines of interdigital type are reverse to each other in contrast to that the directions of electric fields in adjacent intervals between resonant lines of comb line type are the same. Accordingly, when the same coupling coefficient k' is taken, an interval between resonant lines of interdigital type is a, and an interval between resonant lines of comb line type is b. From this fact, it can be said that a microwave band-pass filter of interdigital type is more advantageous than a microwave band-pass filter of comb line type in miniaturization.
So-called stepped impedance type resonant lines in which the width of an open end side of each resonant line is larger than the width on the short-circuit side are disclosed (Japanese Patent Laying-Open No. 62-164301).
FIG. 20 is a diagram showing a microwave band-pass filter employing resonant lines of stepped impedance type disclosed in the above-identified gazette. Referring to the figure, each resonant line 11, 12, 13 includes a short-circuit portion 1c commonly connected to a short-circuit electrode 4 at its one end, an open portion 1a of which one end is open and width is wider than the width of the short-circuit portion 1c, and a connection portion 1b interposed between the open portion la and the short-circuit portion 1c. Also, the microwave band-pass filter includes a guard electrode 5 extending from the short-circuit electrode 4 to the main surface. The guard electrode 5 is formed in order to prevent difference of dimensions of resonant lines and so forth because of up and down movement of a circuit pattern in a length direction when forming a certain pattern on a substrate by the screen printing method, for example.
In the above-described structure, because the open portion 1a is wider than the short-circuit portion 1c, the electrostatic capacity can be made large. Thus, resonant frequency decreases. As a result, as compared to a microwave band-pass filter of resonant frequency same as the decreased resonant frequency, the length of resonant lines can be shorter to reduce size of a dielectric substrate.
However, the shape of the connection portion 1b is step-formed, so that disorder of an electric field and a magnetic field in the discontinuous portion become great, which causes a problem of degradation of a quality factor Q.
Also, for example, when forming a circuit pattern by the screen printing method, since the connection portion 1b is step-formed, an edge of a mask is changed in its form depending on the frequency in use of the mask. As a result, edge portions of connecting portions 1b have variations in shape to cause variations in the resonant frequency.
Furthermore, since capacitance is parasitically produced between the guard electrode 5 and open ends of the resonant lines 11, 12, 13, there is a problem that the capacitance influences the filter characteristic.
Furthermore, there are small differences in permittivity of dielectric substrates A, which produce differences in substantial length of the resonant lines and electrostatic capacitance to influence the filter characteristic.